Conventionally, a temperature sensor has been widely used that uses a thermistor of which the electric resistance value (hereinafter simply referred to as a resistance value) changes according to a temperature, as a heat sensitive body. The characteristics of the thermistor are generally shown by the resistance value and a temperature coefficient of resistance (temperature dependence of the resistance value). The resistance value characteristics of the thermistor are different depending on a material constituting the element, and various materials have been developed which show the resistance value characteristics according to the purpose of use.
An average temperature coefficient of resistance (hereinafter referred to as B constant) can be obtained by the following expression;B=(ln Rm−ln Rn)/(1/Tm−1/Tn)
Rm: Resistance value at temperature Tm
Rn: Resistance value at temperature Tn
The thermistor is a substance which detects a temperature based on a change in the resistance value, and when the resistance value becomes too low, cannot accurately detect the temperature. Accordingly, a thermistor which is used in a wide temperature range is required to have a small B constant.
As is disclosed in Patent Literature 1 and Patent Literature 2, a thermistor is known which has an NTC (Negative Temperature Coefficient: negative temperature coefficient of resistance) characteristics having a B constant of 2500 K or lower in a temperature range of 25° C. to 700° C. It is disclosed that the thermistors in Patent Literature 1 and Patent Literature 2 are formed of an oxide sintered body of which the composition is Y, Cr, Mn and Ca, and that a typical composition of Y, Cr, Mn and Ca is Y: 79.5 mol %, Cr: 8.5 mol %, Mn: 8.5 mol % and Ca: 3.5 mol %.
According to Patent Literature 1, this thermistor sintered body has resistance values of 28.801 kΩ, 6.291 kΩ, 0.6862 kΩ, 0.2196 kΩ and 0.1041 kΩ at 25° C., 100° C., 300° C., 500° C. and 700° C., respectively; and has the B constant of 2417 K in a temperature range between 25° C. and 700° C. It is disclosed that this thermistor can be used for controlling a temperature to 700° C. or lower.
Patent Literature 2 uses a thermistor chip formed of an oxide sintered body having the same composition as that of Patent Literature 1, and achieves a mechanically, thermally and chemically stable thermistor element over a wide temperature range from −50° C. to approximately 1000° C. However, Patent Literature 2 achieves the stable thermistor element in the above described wide temperature range, by covering a joined portion between the thermistor chip and a lead wire, with a covering material that is formed of the metal oxide and a sintering accelerator which does not have a conductivity enhancing function. In other words, a different point between Patent Literature 2 and Patent Literature 1 exists in a material of the covering material, and the B constant of the oxide sintered body which is disclosed by Patent Literature 2 and has the same composition as that of Patent Literature 1 is equivalent to that of Patent Literature 1.